This is my first post here. Not sure if its the right forum area, but I am hoping to get some suggestions on how to go about this strange issue I have. About 10 years ago we had a large tree removed in our back yard and the majority of the stump was removed. We then had stonework done. In the circle planter that is in the same location where the tree used to be we started getting a bad fungi growth that I tried to remove many times over the years. So I finally decided to gut the area, remove the large fungi mound and do it right with some dirt fill , black plastic and then top layer small stone pebbles.

After removing the fungi the area became very loose and started to cave-in in some spots, including where one of the stones fell through about a foot down. When we got the stone installed I could have sworn they installed these stone flat on ground dirt, but now there seems to be some hollowing going on and the only thing I can think of is termites are devouring the left over root system from the tree. My dog was digging and barking around the area one day and he only does that if hes chasing a rodent or something so I am kind of concerned rats are using this as a hideout. The whole area of the stone work used to be flat but now there are some areas where it dips a bit. I was planning on just digging the most I can of this old rotten soggy roots out that I can. It's strange that these chunks of root are very moist as our land tends to be very dry and dusty here in southern California. Its clearly got termites on it (no experience with termites like this before) I was thinking of filling it with dirt. Is that the best action to take? I cant really spray anything poisonous down there to kill the termites because I have a dog. Any suggestions are greatly appreciated thanks.

Hate to say this:

big trees can have enormous root systems .... was a major area of concern when I fought forest fires in the SW....smoldering roots would sometimes create a blast furnace like conditions underground, and be several *feet in diameter* and hundreds of feet long..... one step on a weak spot, and its game over...

that said ...I would start over ....remove all the stonework and dig out any leftover roots ....
sand is a good filler for rotting roots ..will also help in leveling ..... but it may take years for the sand to fill in the voids

Thanks for the response. I guess my main concern is trying to figure out if the tunneling empty space under there is from rats or termites. Rats in the area has been an issue in the past, and if its termites im worried they are going to reach my house which is 20 feet away from that pit, and about 10 feet between that pit and my house is a large dip in the stone where I am guessing is another hollow spot.

If we remove the stone could we use some natural remedies to make the area termite proof? I think you're right that digging out what remains might be the only option.

Silly question (If lets say termites eat all the roots and dont make it to the house would they just die-off because theres nothing left for them to eat?)

Would think you would see some kind of evidence from either termites or rats ....
Rats:
would need escape entrances and exits - and need access to water & food...look for soil excavation in the form of dirt mounds with a hole beside it ......
control - there are wafer like products, that tear off into sticks ....(remove any pets before doing this)... you toss a stick where a rat might find it ...rat eats it - gets super thirsty ...then leaves area looking for water, then dies

Termites:
Cant live without moisture ....so they build "exterior tunnels" that go around not edible to edible food sources ... so do a walk-around of your property ...these tunnels look like saw-dust or dirt or a combo of both and about 1/4" wide or so .... these tunnels always lead back to a source of moisture, like mulch or constantly wet earth .....
Termites "swarm" at least once a year ..... where tens thousands emerge .... and fly off looking for something else to feed on .......
control - cant really help with a natural remedy .... except try to remove any mulch - and fix a possible leaky faucet ....
Would call termite control and get a free inspection and see what they say ....might also get a 2nd or 3rd opinion
have heard some plants can deter termites ...but have no idea how effective it is for an "entire" house

Best case .... the depressions or sinking is just from natural subsidence ....this happens has roots rot, or ground compacts itself under its own weight ...can also happen after heavy rains

I wouldn't pull up all the flagstone. I would continue what you are doing by pulling up the affected stones and backfilling with sand.

I'm more worried about the termites. I think you are right to be concerned about them invading your house so you should find out right away if you have them. You can have a company come out and tell you if you have termites, but a lot of companies will tell you have termites whether you do or not. They are looking to have you pay them to eliminate the termites. Either find a highly reputable company or buy some termite traps. If used correctly the traps will let you know if you have termites.

To keep your pets from getting into poison, you could put termite bait in the empty tree ring and then cover it with a piece of plywood for a couple of months or however long it takes. And everytime you pull up a piece of flagstone, dig down to the hollow area and put some bait down there.

You don't want to use a spray, use a dry bait that the workers will take to the the queen.

Same for the rats, put down poison and cover it up.

If you remove a large tree, you had either best remove ALL, and I do mean all, the roots system or you will be dealing with cavities left by rotting tree roots for decades.
They did not remove anywhere near enough root system and you are simply dealing with a collapsing root system.
If you do not dig it out entirely what ever you put on top will continue to fall or sag for years.
It is moist because rotting roots hold moisture like a sponge.

I am living part time in a 100 year old house that has had many elm trees removed over the decades.
I put paver stones over one spot, a driveway before I reworked it where the former owner said he thought a big elm once stood and no longer used it as as parking spot.
Now the pavers are falling in spots and I have to figure out how much of a root system is falling.
Any tree that was there died over sixty years ago.
Although this is better than a spot on the opposite side of the house where either a former well, or out house, spot in the garden I had one morning left a hole approx. 6 feet across and well over 6 feet deep.

This might sound a little out there, but regarding the rats, know anyone with a good rat terrier? A good ratter can make short work of rats. If I could have a dog, that's the kind of dog I would have.

As to the termites...
We moved into a house with termites and ants. We rent from a landlord whose idea of success is based on their bank account. Long short, it's our problem not theirs unless we want to waste time and money in court and risk eviction.

We tried a lot of things, I forget them now, because what worked the best was wood chips. I put an unconscionable amount of wood chips in the yard as mulch, for the pathways, to rot as compost. About 30 chipper loads the first two years and about two or three a year since. (We have an acre lot.) The ants and termites moved out of the house and now live happily in the wood chips.

I hesitate to make that recommendation, because what works in one place won't necessarily work in another. It could be they moved out because they'd eaten their fill of the house! Who knows!

As to the fungi, not all fungi is bad. That said, I came across of vile smelling fungi in the yard a few years back, and all the plants in the area looked ill. I ripped out the plants because I didn't want them anyhow, then I drowned the area with hydrogen peroxide. No more fungi.

I forgot to mention, rodents, ants, termites, all sorts of folks we'd rather not have love living under stonework, asphalt, and other hardscape that keeps them safe from predators and harsh weather. I used to think it was stupid to see folks watering their driveways (still do under most circumstances.) Then I learned if I watered the crack in the asphalt, the ants would stay in the ground. If I didn't, they'd come into the house looking for water.

Rats are congregated and get settled in garden areas. No matter how small or big your garden is, it can be a habitat for them.

You can find various rats like Brown rat, House mouse, Wood mouse, Field vole and, Bank vole in your garden areas. Plan out rat traps like glue traps, snap traps and bait traps. This is an immediate solution to your problem.
If you are trying to use poisons or toxins for your bait then this might get tricky. Seek professional help from
Roseville Pest Exterminators in CA or you can visit https://www.rosevillecapestcon...

Also make sure you keep your garden clean by removing wood clippings, residues and cutting long grass.
This will help you eliminate the rats in your areas.

I would bet a large sum of money, this is a simple case of the roots of a large tree are rotting, and that is all.
I do not know how large the tree was but multiple roots over a foot in diameter go down over 4 feet, some well over.
All you did, or whom ever did the tree, was remove a little of the top of the tree trunk probably not touching the roots.
I land-scaped for years and never, ever, saw rats live in old tree roots (now maybe this area has a weird rat problem, so I could be wrong but at that , Rats, do not tunnel under ground.)
The ONLY way you are going to solve this problem is to hire some one with a back-hoe to dig down at least 6 feet and remove ALL the old roots.
They will be decaying for most of the next decade, or longer.
I do not know what rodents you have there but I have had them live in old holes in the ground for decades, some of my dogs and cats would stand by the hole and stare or , in the dogs case go bonkers dancing around the hole sticking his nose in.
They are harmless.

For the decaying wood to be moist and crumbling is just the natural case of rotting wood, depending on species of tree, determines how long it takes to rot. ----( I once buried some old Elm root from a Elm that had been gone for years but I dug out, by hand, the then rotting roots down three feet to reduce collapsing soil, some was not lumber hard but far from crumbly. I buried it in the garden about two feet down. When I planted some rhubarb in that are, years later the Elm wood was mostly still there.)
What you have is the main reason if any one cuts down a large tree and does not get rid of
ALL of the roots at least five feet down, may or will , have pain in the arse years on down the line.
Addendum:

At that, where I put in a garden thirty years ago, twenty some back, I woke up one day to find, a hole over six feet deep and five feet wide.

Hiring professional Gold Coast tree loppers or stump removal crew can help with this. Old roots can be a hassle to building a good garden.

deepsouth said:
Hate to say this:
.I would start over ....remove all the stonework and dig out any leftover roots ....

That's super drastic Deepsouth. Yes, Deep is correct.. nothin you can do about the collapse of organic material, but you certainly can decide to pull pavers and top off where needed once a year, rather than digging up the whole yard and trying to remove half rotted roots Which is never gonna be successful.. you're never gonna get it all out anyway, which deep also said No pavement is ever guaranteed to stay level no matter how you prepare the top 2 ft of soil unless you're poring foundations.
Go to your local big box store and buy a 8ft long 2x, check it to make sure it's straight, and pick up$20 of sand. drag your beam around your patio and mark any drastically deep areas with chalk. Pry those areas up, sand them and Replace stone. It's a one day operation

I agree with RpR and Turbosaurus

StreamGrove

I think if i was you i would call a exterminator company.I have had termites and carpenter ants and they can eat a old tree stump like what you are describing.